Book/Dissertation / PhD Thesis

FZJ-2018-06707

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Aging and Degradation Behavior of Electrode Materials in Solid Oxide Fuel Cells (SOFCs)

Yin, X. (Corresponding author)FZJ*

2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-374-7

Please use a persistent id in citations: http://hdl.handle.net/2128/21027  urn:nbn:de:0001-2019020600

Abstract: (La,Sr)(Co,Fe)O$_{3-δ}$ is one of the most potential cathode materials for solid oxide fuel cell(SOFC) applications. Sr in this type of cathode material is very reactive to form secondary phases with other oxides, which affect micro structures and properties of the cathode material, the GDC layer and the ZrO$_{2}$-based electrolyte. The Sr related degradation issues, Cr poisoning and volatile Sr species formation, are studied. As supplement to existing experimental knowledge on Cr poisoning, specific thermodynamic aspects for Cr poisoning are discussed. The thermodynamic calculations show that pCrO$_{3}$ has a stronger temperature dependence than pCrO$_{2}$(OH)$_{2}$, and when considering the reaction between SrO and CrO$_{3}$(g), dependent on different pCrO$_{3}$ and local pO$_{2}$ in the cathode, different Sr-Cr-O secondary phases SrCrO$_{4}$,SrCrO$_{3}$, Sr$_{3}$Cr$_{2}$O$_{8}$ or Sr$_{2}$CrO$_{4}$ could be formed. Additionally, thermodynamic calculations show that in the presence of water vapor, formation of volatile Sr(OH)$_{2}$ is possible as well. pSr(OH)$_{2}$ depends on temperature, pH$_{2}$O and SrO activity, and can be of the same order of magnitude as pCrO$_{2}$(OH)$_{2}$. Volatile Sr(OH)$_{2}$ diffuse through the porous GDC layer and react with ZrO$_{2}$-based electrolytes to form SrZrO$_{3}$ precipitates. The reaction between gaseous Sr species and an 8YSZ sheet is studied experimentally. The surface of the 8YSZ sheet is investigated by SEM coupled with EDS, confirming the deposition of Sr. Since the reaction between the gaseous Sr species and 8YSZ depends on theZrO$_{2}$ activity in 8YSZ, the ZrO$_{2}$ activity in 8YSZ is measured by Knudsen Effusion Mass Spectrometry. The measured aZrO$_{2}$ shows no temperature dependence, which is around 0.85. A high ZrO$_{2}$ activity in 8YSZ facilitates the reaction between the gaseous Sr species and 8YSZ from a thermodynamic point of view. In addition, first principles phonon calculations combined with quasi-harmonic approximation (QHA) are used to predict the thermal expansion of La$_{0.5}$Sr$_{0.5}$Co$_{0.25}$Fe$_{0.75}$O$_{3}$ (LSCF55). Within the framework of the QHA, the volumetric thermal expansion coefficient of LSCF55 is calculated as $\alpha_{𝑉,𝐺𝐺𝐴}$ = 50.34 ∗ 10$^{−6}$ 𝐾$^{−1}$. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in-situ high temperature X-ray diffractometer (HT-XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with $\alpha_{𝑎,ℎ𝑒𝑥}$ = 10.89 ∗ 10$^{−6}$ 𝐾$^{−1}$ and $\alpha_{𝑐,ℎ𝑒𝑥}$ = 21.18 ∗ 10$^{−6}$ 𝐾$^{−1}$ is obtained. The volumetric thermal expansion coefficient is measured as $\alpha_{𝑉,𝐻𝑇−𝑋𝑅𝐷}$ = 43.17 ∗ 10$^{−6}$ 𝐾$^{−1}$. Additionally, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high-performance thermo-mechanical analyzer and yield $\alpha_{𝑙,𝑏𝑎𝑟 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛}$ = 17.37 ∗ 10$^{−6}$ 𝐾$^{−1}$ and $\alpha_{𝑉,𝑏𝑎𝑟 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛}$ = 52.11 ∗ 10$^{−6}$ 𝐾$^{−1}$. A Good agreement between the calculated and measured values of $\alpha_{𝑉}$ is obtained.

Note: RWTH Aachen, Diss., 2018

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Contributing Institute(s):

1. Werkstoffstruktur und -eigenschaften (IEK-2)

Research Program(s):

1. 243 - Tropospheric trace substances and their transformation processes (POF3-243) (POF3-243)
2. 135 - Fuel Cells (POF3-135) (POF3-135)

Appears in the scientific report 2018

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Database coverage:
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